(a) Field of the Invention
The present invention relates to a liquid crystal display and a drive method thereof. More particularly, the present invention relates to a liquid crystal display and a drive method thereof in which swinging of a common electrode voltage is performed by synchronizing the voltage with a gate pulse, thereby generating an overshoot to improve a response speed of liquid crystal material.
(b) Description of the Related Art
In recent times, there has been an ever-increasing need for lighter and thinner displays. Accordingly, the liquid crystal display (LCD) is replacing the CRT in many applications such as in television and as displays for personal computers.
LCDs include first and second substrates provided substantially in parallel with a predetermined gap therebetween, and liquid crystal material is sandwiched between the two opposing substrates. A voltage is applied to the liquid crystal material, which has a dielectric anisotropy, to form an electric field between the substrates. By varying the strength of the electric field, the alignment of liquid crystal molecules of the liquid crystal material is controlled to thereby control the transmittance of incident light. Although there are different types of LCDs, the most type is the thin film transistor (TFT) LCD.
FIG. 1 shows a pixel equivalent circuit of a conventional TFT-LCD. With reference to the drawing, a pixel of the TFT-LCD includes a TFT switching element in which a source terminal and a gate terminal are respectively connected to a data line and a gate line; a liquid crystal capacitor Clc and a storage capacitor Cst, each connected to a drain terminal of the TFT switching element; a parasitic capacitor Cgd provided between the gate terminal and the drain terminal; a parasitic capacitor Cds connected between the drain terminal and the source terminal; and an overlap capacitor (Cover) provided between the data line and a pixel electrode Vp.
The method of driving the liquid crystal material between the pixel electrode, which is provided on a TFT substrate, and a common electrode which is provided on a color filter substrate, will now be described.
First, if a positive pulse is applied through the gate line, the TFT switching element turns on. At this time, a signal voltage applied to a source electrode of the TFT switching element through a signal line is applied to the liquid crystal capacitor Clc and the storage capacitor Cst through the drain terminal. The signal voltage, which is applied together with a gate pulse, is continuously maintained even when a gate voltage is turned off and applied to the liquid crystal capacitor Clc. However, because of the parasitic capacitor Cgd between the gate terminal and the drain terminal, a pixel voltage undergoes a voltage level shift by a certain amount of voltage.
The biggest limitation in the TFT-LCD that is structured and operating as described above is its response speed. Matsushida Company improves a capacitive coupled driving (CCD) mechanism in order to increase the response speed of the TFT-LCD.
FIG. 2 shows a chart for describing the effects of CCD. A direction applied to a pixel for controlling overshooting and undershooting is determined by the liquid crystal property of low anisotropy. If a pulse is applied to a common electrode, an amount that undergoes capacitive coupling increases in the pulse direction of a low anisotropic state of the liquid crystal material. If a pulse in which the voltage is first increased then decreased is applied in the case where the direction applied to the common electrode inverts from positive (+) to negative (−), or if a pulse in which the voltage is increased then decreased is applied in the case where the direction applied to the common electrode inverts from negative (−) to positive (+), conversion from a high gray level to a low gray level occurs for a normal white mode. However, if conversion from the low gray level to the high gray level occurs, undershooting and overshooting of the voltage result such that the liquid crystal material rotates more quickly.
FIG. 3 shows a pixel equivalent circuit of a TFT-LCD using a previous gate disclosed by Matsushita Company, and FIG. 4 shows charts used to describe the increase in response speed for the TFT-LCD of FIG. 3.
With reference to the drawings, one end of a storage capacitor Cst is connected to a drain, and its other end is connected to a previous gate. During operation, a gate pulse is applied such that an average voltage Vp applied to the pixel results as shown in Equation 1.Vp=+Vs+[Cst/(Cst+Cgd+Clc)]ΔVg  [Equation 1]
where Vs is a voltage applied to a source terminal, Cst is a capacitance of the storage capacitor Cst, Cgd is a parasitic capacitance between a gate terminal and a drain terminal, Clc is a capacitance of a liquid crystal capacitor, and ΔVg is a difference between a previous gate voltage and a present gate voltage.
However, the use of the previous gate in the TFT-LCD disclosed by Matsushida Company increases the gate load and can only be applied to line inversion driving. It also generates crosstalk and flicker to thereby make large-scale high resolution difficult. Also, a conventional gate tap IC is not able to be used in the Matsushida TFT-LCD. Further, if the gate voltage excessively increases when turned off in this prior TFT-LCD, an off current increases such that there is a limit to the degree at which a value of the gate can be changed.
As described above, in the TFT-LCD disclosed by Matsushita Company, although the use of a previous gate signal and the drive method of applying a gate signal in two steps significantly improves response speed, there are limits to the application to a large-scale high resolution LCD when considering the use of a previous gate, and line inversion.